Been Constipated Two Weeks Ago Then I Dranked A Lot

Been constipated two weeks ago then I dranked a lot of water every day but nothing happened. Then I pooped very hard all of a sudden to let the poop come out. Afterwards my head hurts and I tried drinking a lot of water. But nothing happened and I'm also underweight too. I'm just too scared to eat since my constipation and I've been eating slowly ever since. Should I take a laxative?